This invention relates to a fuse element used with an integrated memory device, and more particularly to the construction of connecting portions integrally formed at both ends of a melting away portion of the fuse element.
A process which is generally known is shown in FIG. 1 and comprises the step of inserting fuse elements between the row and column lines of a read only memory (ROM) at their junctions through a gate-emitter pass of a transistor, and, when the ROM is programmed, cutting a prescribed fuse by means of, for example, a laser beam. FIG. 1 typically indicates a transistor TRn.multidot.m whose base electrode is connected to a row line Yn, whose collector is connected to a power source Vcc, and whose emitter is connected to a column line Xm through a fuse element F1, and a transistor TR(n+1)m whose base electrode is connected to a row line Yn+1, whose collector is connected to the power source Vcc and whose emitter is connected to a column line Xm through a fuse element F2. FIG. 1 illustrates the case where the fuse element F2 is thermally broken off. Transistors TRn(m+1), TR(n+1)(m+1), and fuse elements F3, F4 are connected to the row and column lines in the same manner as described above.
It is also known that a random access memory (RAM) is provided with a redundancy circuit comprising a spare row or column section, and when any row or column section of the RAM fails, the defective row or column section is replaced by the corresponding spare row or column section through the fuse element. The fuse element is generally prepared from, for example, aluminium, nichrome or polycrystalline silicon. This kind of fuse element has hitherto been made in the form shown in FIG. 2A or 2B, namely, comprising a melting away portion 12a which is expected in case it needs to be thermally broken off, and connecting portions 12b integrally formed at both ends of the melting away portion 12a. Both connecting portions 12a, 12b are integrally prepared from, for example, a polycrystalline layer. FIGS. 3A and 3B indicate the position of the fuse element comprising the aforesaid melting away portion 12a and connecting portions 12b (not shown) relative to a semiconductor substrate 13 and an insulation layer 14. A semiconductor device or the integrated memory (not shown) is mounted on that portion of the semiconductor substrate 13 which is not indicated in FIGS. 3A and 3B. The insulation layer 14 prepared from, for example, SiO.sub.2 is deposited on one surface of the semiconductor substrate. FIG. 3A is a sectional view of a semiconductor device indicating a melting away portion 12a of the fuse element mounted on the insulation layer 14. FIG. 3B is a sectional view of a semiconductor device having an insulating protective layer 15 enclosing the melting away portion 12a of the fuse element of FIG. 3A.
Description will now be given of the drawbacks encountered in the conventional fuse element of FIGS. 2A and 2B which were prepared from, for example, polycrystalline silicon. SiO.sub.2 has a thermal diffusion coefficient of 8.4.times.10.sup.-3 cm.sup.2 /s, while a fuse element prepared from polycrystalline silicon has prominently large thermal diffusion coefficient of 0.5 cm.sup.2 /s. A wire prepared from, for example, polycrystalline silicon is fitted to the connecting portions 12b of the fuse element, and further to a semiconductor device or the integrated memory (not shown) formed on the semiconductor substrate 13. When the melting away portion 12a of the fuse element is thermally broken off, heat energy is easily conducted to the semiconductor element of the semiconductor device through the connecting portions 12b and polycrystalline silicon wire. Consequently, the semiconductor element is readily damaged by the heat energy received. The above-mentioned drawbacks may be summarized as follows: (1) The heat energy produced in the melting away portion 12a of the fuse element sometimes dissolves a wire connected to the connecting portions 12b or an aluminium electrode of a semiconductor element ccnnected to the wire; (2). When the connecting portions 12b of the fuse element are fitted to a highly resistive polycrystalline silicon layer which is not doped with any impurity, then an external impurity is likely to be diffused in said highly resistive layer, thereby causing the resistivity of the highly resistive layer to be reduced to an undesirable low level; (3). A leak current tends to flow, due to the conduction of heat energy, through a PN junction of a semiconductor element formed on the semiconductor substrate 13 in the proximity of the fuse element; and (4). Heat energy released from the melting away portion 12a of the fuse element causes, for example, Na.sup.+ ions included in the surface of the insulating layer 14 to be readily diffused, causing the electric properties of the semiconductor element to be easily changed due to the diffusion of the Na.sup.+ ions. The conventional process of suppressing the drawbacks described in the above-noted items (1) to (4) comprises means for extending a distance between the fuse element and semiconductor element as much as possible, and means for enlarging the area of the connecting portions 12b of the fuse element to increase the heat capacity thereof, thereby preventing heat from being released from the connecting portions 12b. When raising the integration circuits density of integrated particularly large scale integrated circuit, the two above-mentioned means obviously prove unsatisfactory. Consequently, it has been strongly desired to develop means which can reduce an area occupied by a fuse element and eliminate the drawbacks of the conventional fuse element described in the items (1) to (4).